The celebrity face of bipolar disorder has typically been, well, sort of crazy (think Britney Spears or, so it’s been speculated, Charlie Sheen). So when a publicist for Catherine Zeta-Jones — who has been stalwart in supporting her husband, Michael Douglas, through his battle with cancer over the past year — announced that the actress had sought treatment for bipolar 2, it was a surprise to many.

“After dealing with the stress of the past year, Catherine made the decision to check in to a mental health facility for a brief stay to treat her bipolar 2 disorder,” publicist Cece Yorke said in a statement. (More on Time.com: Do We Really Need Psychiatrists to Do Therapy?)

A friend of the actress toldPeople Magazine that Zeta-Jones spent five days in the undisclosed facility. “Catherine has had to deal with Michael’s illness and that’s been hard,” the friend said. “She went in for a few days because she’s about to start working, and wanted to make sure she’s in top form, which she is.”

In announcing her illness, Zeta-Jones sheds light on the fact that many people are able to manage a successful career and a family life, along with their disorder. She also reminds us that bipolar disorder, which affects 2% to 3% of the U.S. population, exists on a spectrum, and that some forms of the illness are more severe than others.

In bipolar 1, the most severe form, patients typically swing between distinct episodes of depression and mania (though some may experience both manic and depressed symptoms simultaneously), with periods of normal mood in between. During manic episodes, which may last anywhere from a week to a few months, patients experience abnormally high mood, agitation, irritability and grandiosity. They have racing thoughts, usually need very little sleep and act impulsively, having reckless sex or overspending, for instance. (More on Time.com: Is Dr. Drew More Like Charlie Sheen than He Thinks?)

During episodes of depression, which usually last longer than manic episodes — at least two weeks, but sometimes longer than a year — patients feel sad, hopeless and worthless. They have trouble concentrating or making decisions. They suffer disordered sleep and eating, and lose interest in participating in daily activities.

Some bipolar patients also suffer from psychosis, including hallucinations and delusions. But with proper treatment, patients can enjoy long periods of stable behavior.

Bipolar 2 is less severe than bipolar 1. The key difference is that patients with bipolar 2 do not have full-blown manic episodes. Rather, they cycle between milder, hypomanic episodes and episodes of major depression. Women are slightly more likely to have bipolar 2 than men.

“One of the things people often overlook because manias and hypomanias are splashy, is that most patients with bipolar disorder spend their lives depressed,” says Dr. Martin Evers, an outpatient psychiatrist and associate director of behavioral health at Northern Westchester Hospital. “The tragedy of the disorder is the depression. A lot of days of your life are lost.” (More on Time.com: Addict or Bipolar? Examining the ‘Passion’ of Charlie Sheen)

For that reason, many bipolar adults — particularly those with bipolar 2 — are misdiagnosed with clinical depression. “People tend to seek treatment for depression, but not for mania because it feels good,” Evers says.

Other forms of bipolar disorder include bipolar disorder not otherwise specified — a diagnosis that describes patients whose illness does not fit into other categories — and cyclothymic disorder, whose sufferers are typically seen as extremely moody but whose mood swings are not considered severe or lasting enough to qualify as bipolar.

Untreated bipolar disorder usually worsens over time, with episodes becoming more severe and more frequent. “Rapid cyclers” — about 20% to 25% of patients — have four or more episodes of major depression, mania, hypomania or mixed symptoms within a year. Some people experience more than one episode in a week, or even within a single day. Rapid cycling appears to be more common in people with severe illness and in those whose disease developed earlier, during their mid to late teens. Typical onset of the disorder is in the 20s. (More on Time.com: Why Has Childhood Bipolar Disorder Become an Epidemic?)

It remains unclear when Zeta-Jones received her diagnosis, but it is important to note that many patients may go decades believing they are depressed, without realizing they have bipolar disorder.

With proper treatment, however, most people with bipolar disorder can largely control their mood swings and related symptoms, reducing their severity and relapse. Because it is a lifelong and recurrent illness, people need long-term treatment including both medication and psychotherapy, according to the National Institute of Mental Health.